It took me two days to do it.
I used Particle Illusion, which is very easy (I found...) to make fireballs
First I import avi of my scenes in ParticleIllusion, to have a layer-background
I make fireballs etc.
I export the scenes in TIF format with alpha layer
And I import all images in AfterEffect in a new sequence
I put this sequence over my movie, and that's all